(a) Field of the Invention
The present invention concerns in general terms a method of obtaining an antenna gain function. More particularly, the present invention relates to a method of obtaining an antenna gain for a base station in a mobile telecommunication system. It makes it possible to obtain an antenna gain function, in transmission or reception mode, which is invariant by change of frequency.
(b) Description of the Related Art
The formation of channels or the elimination of interfering signals is well known in the field of narrow-band antenna processing. Both of these use an array of antennae, generally linear and uniform (that is to say with a constant pitch) and a signal weighting module. More precisely, if it is wished to form a channel in reception mode, the signals received by the different antennae are weighted by means of a set of complex coefficients before being added. Conversely, if it is wished to form a channel in transmission mode, the signal to be transmitted is weighted by a set of complex coefficients and the signals thus weighted are transmitted by the different antennae.
FIG. 1 illustrates a known device for obtaining antenna gain in transmission and reception mode. The device comprises an array of antennae (100),(101), . . . ,(10N−1), a transmission weighting module (11) and a reception weighting module (15). The signals received by the different antennae, (xi), i=0 . . . N−1 are weighted at (130),(131) . . . ,(13N−1) by a set of complex coefficients (bui), i=0, . . . ,N−1 before being added at (14) in order to give a signal Ru. Conversely, a signal to be transmitted Sd is weighted at (120),(121) . . . ,(12N−1) by means of a set of complex coefficients (bdi), i=0, . . ,N−1, before being transmitted by the different antennae.
If respectively the vector of the signals received and the vector of the weighting coefficients is denoted {overscore (x)}=(x0,x1, . . . ,xN−1)T and {overscore (bu)}=(bu0,bu1, . . . ,buN−1)T, it is possible to write:Ru={overscore (b)}u{overscore (x)}  (1)
The complex gain (or the complex gain function of the antenna) in reception mode can be written:
                              G          ⁡                      (                                                            b                  u                                _                            ,              θ                        )                          =                                                                              b                  u                                _                            T                        ·                                                            e                  u                                ⁢                θ                            _                                =                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          b                ui                            ·                              exp                ⁡                                  (                                      -                                          jφ                      i                                                        )                                                                                        (        2        )            
where euθ represents the vector {overscore (x)} corresponding to a flat wave arriving at an angle of incidence θ, andφi=(2πd/λ).i.sin(θ)=(2πdf/c).i.sin(θ)  (3)
is the difference in operation between consecutive antennae for a uniform linear array of pitch d, λ and f being respectively the wavelength and the frequency of the flat wave in question;φi=2πRΔθ/λ.sin(θ−θi)=2πRfΔθ/c.sin(θ−θi)  (4)
for a circular array where θi is the angle between a reference axis and the normal to the antenna of index i, R the radius of curvature of the array, Δθ is the angular difference between two consecutive antennae in the array.
Likewise the complex gain (or the complex gain function) in transmission mode can be written:
                              G          ⁡                      (                                                            b                  d                                _                            ,              θ                        )                          =                                                                              b                  d                                _                            T                        ·                                                            e                  d                                ⁢                θ                            _                                =                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          b                di                            ·                              exp                ⁡                                  (                                      jφ                    i                                    )                                                                                        (        5        )            with the same conventions as those adopted above and where {overscore (edθ)} designates the vector {overscore (x)} corresponding to a flat wave transmitted in the direction θ. The weighting vectors in reception and transmission mode respectively will be called {overscore (bu)} and {overscore (bd)}.
Clearly, the antenna gain in transmission or reception mode depends on the frequency of the signal in question. There are however many situations in which the antenna gain must remain unchanged whatever the frequency of the signal. For example, in so-called FDD (Frequency Division Duplex) mobile telecommunication systems, where the frequency used on the downlink, that is to say from the base station to the mobile station, differs from that used on the uplink. Similarly, in frequency-hopping radar systems, it is necessary to ensure the invariance of the gain function, notably in order to aim a transmission or reception beam in a given direction or to eliminate the interference coming from a given direction, whatever the frequency used.
In more general terms, it is desirable to be able to obtain, for a given signal frequency, an antenna gain function which is as close as possible, in the sense of a certain metric, to a reference gain function. The reference gain function can notably be a gain function obtained at a given frequency which it is sought to approximate to the greatest possible extent during transmission or reception at another frequency.